On the possibility of compensating material dispersion in three-layer optical fibres in the wavelength range below 1.3 μm

Idealised three-layer model optical fibres consisting of an optically dense core with the refractive index n₁ surrounded by a 'holey' intermediate cladding having a much lower optical density and the refractive index n₂ and by a rather thick outer cladding with the refractive index n₃ are investigated. The idealised model assumes that the intermediate cladding is homogeneous. It is also assumed that n₁ > n₃ > n₂. It is shown that in such fibres with large differences n₁ – n₂ and n₃ – n₂, a single-mode regime can be realised in fact, when the nearest higher modes H₀₁, E₀₁, HE₂₁ have additional radiative losses of several hundred dB km^-1, whereas the main operating mode HE₁₁ has no such losses in the single-mode regime. It is important that the zero dispersion can be obtained in these fibres in the spectral region below 1.3 μm.